This invention relates to polymer foam materials made from blends of an ethylenic polymer and a styrenic polymer, and more particularly to a foamable blend of a polyethylene and a polystyrene lightly crosslinked by free radical generating compounds.
Both polystyrene foams and polyolefin foams have heretofore been used for a variety of purposes. Polystyrene foams are lightweight and have both rigidity and good shape retention. However, polystyrene foams are not resistent to solvents and are deficient in certain important properties such as flexibility, compressive recovery, and capacity to absorb repeated impacts. Polyolefin foams, while having flexibility and good impact absorbing properties, have generally suffered from low thermal resistance and have been subject to creep. It would be disirable to blend the two resins to produce foams having the advantageous properties of each.
Prior efforts to blend these two types of polymers have had to face the threshold problem that olefin and styrenic polymers are generally incompatible. For example, British Pat. No. 1,460,621 teaches a foam prepared from a blend polyethylene and polystyrene, combined with a block copolymer of styrene and butadiene or a graft polymer of styrene monomer onto polyethylene. The block or graft polymers are added to render the two other resins more compatible.
Japanese Patent Application No. 55/181,384 also teaches a foam which is a blend of polyethylene and polystyrene. However, the resin blend is foamable only for certain specified ratios of melt indices of polystyrene to polyethylene, and for certain polystyrene to polyethylene weight ratios.
Finally, Kannan et al, U.S. Pat. No. 4,262,052, teach a foamed composite material, one component thereof being a blend of a polyolefin, a vinyl aromatic polymer such as styrene, and a graft polymer formed by graft polymerizing a vinyl aromatic monomer onto the backbone of the polyolefin. A crosslinking agent such as an organic peroxide aids the grafting reaction.
However, all of these previous efforts to blend polyolefin and polystyrene resins to produce foams have either been complicated by the need for a separate reaction sequence to form a block or graft copolymer to add to the blend or have been limited to certain melt index or weight ratios of the starting resins. For example, the previously discussed Japanese Patent Application No. 55/181,384 utilized a very viscous, low melt index polyethylene. Such a viscous polymer causes pumping, pressure, and mixing problems in conventional melt processing equipment such as extruders.
Accordingly, the need exists in the art for a composition blending together these two types of resins so that they can be foamed readily by conventional melt processing techniques.